Substances that are usually actively transported are sodium , potassium, calcium, urate ions , hydrogen ions , chloride ions & most of the aminoacids. Carrier proteins have a major role in active transport .

Function :
1. It is the basis of nerve function for transmitting nerve impulses.
2. Keeping the interior of cell negatively charged & exteroir positively charged.
3. Control of the volume of the cell : Proteins & other organic compounds present in the interior of the cell is negatively charged . So they have a tendency to attract positively charged particles from outside along with water molecules. Once the cell swells , it activate the sodium – potassium pump & which help in maintaining the volume of cell intact .

Through the sodium – potassium pump sodium ion is transported to the exterior of the cell & potassium ions to the interior with the help of carrier protein . This carrier protein has three sodium ion binding sites , two potassium ion binding sites & ATPase activity which help in splitting up of ATP to ADP & inorganic phosphate , inturn release energy .

Calcium pump
Calcium is transported from ICF to ECF .
Hydrogen ion transport
Occurs in,
1. Gastric glands of the stomach especially with parietal cells & is the basis of HCl secretion in stomach
2. In late distal tubule & cortical collecting tubules of the kidney by excreting H+ ion in urine & to maintain the appropriate H+ concentration in blood .

Applied physiology
1. Na/ K ATPase is inhibited by cardiac glycoside , digitalis which causes Na+ accumulation inside the cell , inturn causes Ca2+ accumulation in the myocardial cells & this increases myocardial contractility So digitalis is used in a failing heart.
2. Refrigeration of blood at 40C suppresses Na/ K ATPase activity . At 450C – denaturation of enzyme , Optimum activity is at normal body temperature
Secondary active transport
Energy is produced by movement of one solute down concentration gradient is used for the transport of another solute against concentration gradient. ( energy is produced by maintaining an ionic concentratoin gradient by primary active transport )

It is of two types
1. Co- transport
2. Counter transport

Co – transport
e.g.; Glucose & aminoacid co – transport with Na.
Concentration gradient produced by primary active transport represents a store house of energy . There is always a tendency to move Na+ down the concentration gradient to the cell .Under appropriate condition this energy pull certain substance along with Na ions with the help of carrier protein to the interior of cell .

Counter transport
Substance to be transported is inside the membrane . Na+ binds to carrier protein at its exterior surface & the other substance to the interior of carrier protein . Confirmational change occure in the carrier protein & Na+ is transported to the interior of the cell & other substance is transported to the exterior .
e.g.; Na+ / Ca2+ counter transport
Na+ / H+ counter transport